The present invention generally relates to current sensing circuits, and more particularly to a common source current sensing circuit integrated with a trench power MOSFET.
In many power MOSFET applications, monitoring a large current flowing in a load is accomplished by a current sensing element. The current sensing element may include a transistor having a much smaller number of cells than the number of cells in the power MOSFET whose load current is being sensed. The ratio of the number of cells in the current sensing element to the number of cells in the power MOSFET may be on the order of 1:1 million cells.
A conventional current sensing circuit is disclosed in U.S. Pat. No. 4,553,084 entitled “Current Sensing Circuit” to Wrathall. With reference to FIG. 1, the disclosed circuit includes an MOS transistor 11 having its source coupled to supply voltage 12, which for example, may be ground. MOS transistor 13 has its source coupled to supply voltage terminal 12 by sense resistor 14. The drains of both transistors 11, 13 are coupled to supply voltage terminal 15 by load 16. Input node 10 receives a load current from load 16. Gate drive 17 provides a voltage Vg to the gates of transistors 11, 13.
Amplifier 18 has a first input terminal 19 connected to the source of transistor 13 and a second input terminal 21 connected to a reference voltage terminal 22. The output of amplifier 18 is connected to output terminal 23. The output signal from output terminal 23 provides an indication of the load current through load 16 exceeding a predetermined limit. The output signal from output terminal 23 may be provided as feedback to gate drive 17 for performing a current limiting or constant current function. Current flow through transistors 11, 13 is in proportion to the number of cells in each of transistors 11, 13.
The Wrathall scheme is a common drain scheme and is inherently inaccurate. In order for the amplifier 18 to sense reliably, the voltage developed across the sense resistor 14 is typically on the order 0.5V. This voltage across the sense resistor 14 reduces the Vgs of sensing transistor 13 by about the same amount. Hence transistors 11, 13 are operating under different Vgs conditions. With reference to FIG. 2, a difference in current flow through each single cell of two identical transistors having different applied Vgs is shown. As shown, the difference in current flow increases with increasing Vds. When using a transistor with a smaller number of cells as the sensing transistor 13 to sense the current through a transistor 11 having a larger number of cells, the sensed current will deviate from the actual current as shown in FIG. 3.
A more accurate approach employs a common-source configuration as shown in FIG. 4. A common source sensing circuit includes a MOS transistor 40 having its source coupled to supply voltage 42, which for example, may be ground. MOS transistor 43 also has its source coupled to supply voltage terminal 42. The drain of transistor 43 is coupled to supply voltage terminal 45 by sensing resistor 46. Gate drive 30 provides a voltage Vg to the gates of transistors 40, 43. In this configuration, the voltage developed across the sense resistor 46 will not affect the Vgs of the sense transistor 43.
Amplifier 48 has a first input terminal 47 connected to the drain of transistor 43 and a second input terminal 41 connected to a reference voltage terminal 44. The output of amplifier 48 is connected to output terminal 49. The output signal from output terminal 49 provides an indication of the sensed current through resistor 46 exceeding a predetermined limit. The output signal from output terminal 49 may be provided as feedback to gate drive 30 for performing a current limiting or constant current function. Current flow through transistors 40, 43 is in proportion to the number of cells in each of transistors 40, 43.
In the common source configuration transistors 40, 43 operate on the same Vgs curve. Thus the problem shown in FIG. 2 with reference to the common drain scheme disclosed by Wrathall is eliminated.
In standard CMOS design, the common source sensing circuit can be integrated easily into the same power IC chip. For higher performance trench power MOSFET designs, the drains of every cell are connected together making it more difficult to achieve such integration.
The present invention provides for a unique device and packaging design which integrates the common source sensing circuit into a trench power MOSFET device.